Switching operations of switching elements of a power conversion apparatus may cause a high voltage change rate (dv/dt) and a high current change rate (di/dt). A rapid change of a voltage and a rapid change of a current may cause common mode noise through a distributed capacitance between a system and the ground. The common mode noise flows into a grid, which will interfere with other electronic devices connected to the grid.
FIG. 1 illustratively shows a circuit diagram of a typical three-phase power converter topology and reasons for producing common mode noise. Reference number 11 indicates a power conversion unit which for example consists of three semiconductor bridge arms in parallel, and each bridge arm consists of two semiconductor devices. FIG. 1 shows, respectively, DC bus capacitors CB1 121 and CB2 122; DC bus midpoint O 123, i.e. a connection point of the DC bus capacitors CB1 and CB2; an alternative current (AC) source 15 such as a grid. Filter inductors La 131, Lb 132 and Lc 133 are shown respectively. A terminal of each filter inductor is connected to a corresponding phase line of the AC source 15, the other terminal of the inductor is connected with a corresponding midpoint of a bridge arm of the power conversion unit 11, and usually a distribution of the three filter inductors is symmetrical. Reference number 14 indicates a set of star-connected differential mode filter capacitor bank which includes for example three capacitor Cxs that are star-connected, and reference number 141 represents a common terminal N formed by connecting the three capacitor Cxs together. In FIG. 1, filter inductors La, Lb, Lc and X capacitor bank 14 constitute a differential mode filter. G represents the ground which for brevity is indicated only in one place, and other places are identified by the same device symbol. Reference number 16 represents Line Impedance Stabilization Network (LISN) which is an auxiliary equipment for testing conducted electromagnetic interference.
There exists a stray capacitance C0 (100) between the DC bus midpoint O and the ground. Similarly, between midpoints A, B, and C of the bridge arms of the power conversion unit 11 and the ground there exist distributed stray C1a (101), C1b (102) and C1c (103) respectively. Potential jumping of the DC bus midpoint O as well as the midpoints of the bridge arms A, B and C relative to the ground may cause displacement current through the aforementioned stray capacitances C0, C1a, C1b and C1c, and the displacement current flows into the ground and produces common mode noise.
In order to meet international EMC standards, how to suppress common mode noise effectively with low cost is a common concern in the field.
FIG. 2 illustratively shows a circuit diagram of a solution for suppressing common mode noise in conventional technique. FIG. 2 differs from FIG. 1 in that a passive common mode filter 17 is added on three-phase power supply lines between the grid 15 and the power conversion unit 11, to suppress common mode noise. Passive common mode filter 17 in FIG. 2 includes a common mode inductor LCM (171) and a set of Y-capacitor bank 172 which includes, for example, three capacitor CYs that are star-connected, and a common terminal NY (173) of the three capacitor CYs is connected to the ground.
However, the common mode inductor 171 in FIG. 2 is often of large size, and of high cost. Furthermore, when a large induction of a common mode inductor is required, it is even difficult to design the common mode inductors 171.
FIG. 3 illustrates another solution for suppressing common mode noise in conventional technique, that is, to reduce requirements on a common mode filter by reducing original common mode noise. FIG. 3 illustrates an exemplary circuit diagram of such a solution for suppressing common mode noise. FIG. 3 differs from FIG. 1 in that a common terminal N (141) of the star-connected X capacitor bank 14 is connected directly to the DC bus midpoint O (123). Since N is a virtual neutral point whose potential is relatively stable, after the DC bus midpoint O is directly connected to the virtual neutral point N, a potential of the bus relative to the ground is also clamped to a stable potential, which can reduce common mode noise to a certain extent.
However, the solution of FIG. 3 can only suppress common mode noise caused through a stray capacitance C0 (100) between the DC bus of the power converter and the ground, while common mode currents caused by stray capacitances C1a (101), C1b(102), and C1 (103) between the midpoint A, B and C of the bridge arms of the power conversion unit and the ground is far from being suppressed, but increases.